This application claims the priority of United Kingdom Application No. 9805419.0 filed Mar. 14, 1998, which is incorporated herein by reference.
The present invention relates to the treatment of liquids, and more particularly to the treatment thereof so as to improve them through processing, e.g. inactivation or decomposition, of materials dissolved or suspended therein.
Electrical discharge is widely known in gases with various practical applications e.g. for lighting, cutting through metals etc. It is also known in principle that such an electrical discharge can be obtained inside a liquid. To date though this has not found any significant practical application due to inter alia the very short working life of the electrodes in the devices used.
It is an object of the present invention to avoid or reduce one or more of the above disadvantages.
The present invention provides an apparatus suitable for use in the treatment of a liquid, said apparatus comprising a high voltage electric discharge device with a first electrode and a second electrode spaced apart from said first electrode, said discharge device including electrical power supply control means formed and arranged for applying a series of high voltage pulses across said first and second electrodes in use of the apparatus with said device connected to an electrical power supply so as to produce, in use of the apparatus, an extended area electrical discharge in an electric discharge zone extending between first and second electrodes, said apparatus including a gas delivery means formed and arranged for feeding a series of bubbles of gas into a body of liquid in said electric discharge zone in use of said apparatus.
Thus with an apparatus of the present invention it is possible to generate an electrical discharge inside a liquid for treatment of the liquid, with a relatively low voltage electrical power supply and in a way which results in substantially increased working life of the discharge device, with associated economy of construction, etc. Without in any way wishing to restrict the scope of the present invention, it is believed that a significant factor in the reduced voltage threshold required to establish the electrical discharge within the liquid in use of apparatus of the present invention is the generation of electrical discharge within the bubbles of gas passing between the electrodes.
The generation of a pulsed electrical discharge inside a liquid can induce various different and more or less powerful effects in the liquid depending on the form and type of apparatus of the invention used, which can have a strong effect on various materials present in the liquid. In general, within the immediate vicinity of the plasma discharge there are generated various more or less active species such as hydrated electrons, ions, excited atoms, and free radicals, which can react with materials present in the liquid. Zones of liquid extending beyond the immediate vicinity of the plasma discharge can be affected by UV-radiation. In the case of water containing trace amounts of material such as may be encountered in water being treated for domestic consumption, such zones may extend up to 6 cms or more. The extent of such zones will naturally depend on various factors such as the intensity of the plasma discharge which in turn may depend to a greater or lesser extent on such factors as the voltage of the pulses used for the electrical discharge; and on the OD (optical density or absorbance of the liquid and any material dissolved or suspended in it). Acoustic and shock waves may also be generated by the plasma discharge. By using suitable geometric arrangements of secondary electrodes e.g. generally toroidal, it is possible to combine and concentrate such shock waves for even greater cumulative effect. It will moreover be appreciated that unlike with conventional UV-irradiation apparatus, various embodiments of the apparatus of the present invention will have significantly more powerful effect from the UV-irradiation alone due to the avoidance of any barriers to UV-transmission between the source and the liquid being treated. Even quartz glass which has above average UV-transmission characteristics, substantially attenuates UV-radiation with a wavelength of less than 200 nm. In addition, in at least some forms, the present invention can provide more or less significant synergistic effects through the combination of the UV-radiation, acoustic/shock waves generated, and plasma-chemical and photo-chemical reactions. This may be particularly significant in the treatment of liquids containing micro-organisms such as bacteria, so as to kill or inactivate these in order effectively to sterilise the liquid. The apparatus of the present invention may also be used to treat particular resistant infectious agents such as protozoal oocysts and prions.
Various liquids may be treated by use of the present invention, but the apparatus of the present invention is especially useful for the treatment of water contaminated with various different kinds of materials in order to improve the quality of the water, reduce environmental pollution etc. Thus the present invention may be used by public utility companies for the treatment of water supplied for consumption and running pools, and other uses, and for the treatment of waste water. Other liquids that may be mentioned as suitable for treatment include oils, for example used or waste transformer oil containing more or less toxic additives (e.g. PCBsxe2x80x94polychlorinated biphenyls) required for improving its performance, which require to be neutralised to facilitate safe disposal of the oil. Thus the present invention may also be used to decompose hazardous or toxic organo-chemicals into more or less safe and/or easily biodegradable substances. Other organochemicals which may be mentioned as suitable for neutralisation to a greater or lesser extent by means of the present invention include alkanols e.g. methanol; aldehydes e.g. formaldehyde; phenol and substituted phenols e.g. para-chlorophenol; etc. Other applications of the present invention which may be mentioned include activation of oils and generation of ultrasound for medical purposes.
It will be appreciated that various configurations and geometries of electric discharge device and of the electrodes used therein may be used in accordance with the present invention. In a first preferred form of the invention, the first and second electrodes are substantially spaced apart and there is provided a multiplicity of spaced apart secondary electrodes distributed between opposed portions of said first and second electrodes. Conveniently there is used a device form which has a generally annular electric discharge zone, which may be defined by an internal or external cylindrical surface, and having a longitudinal axis which may be substantially rectilinear, or arcuate to a greater or lesser extent e.g. circular (corresponding to a toroidal discharge surface) or helical, and extending between generally annular opposed portions of the first and second electrodes. With such non-rectilinear axis electric discharge zones it will be appreciated that there may be obtained a degree of interaction between shock or acoustic waves generated by the electric discharge, for example, to produce a more focused and intense cumulative effect.
In the above described first form of the invention, the first electrode may also have an elongate portion extending generally parallel to the electric discharge surface and insulated therefrom by a suitable solid insulator dielectric. Thus, for example, with an external electric discharge surface, said first electrode elongate portion could be in the form of a conductor extending generally along the central longitudinal axis of the electric discharge surface. Conveniently the electric discharge surface is provided on the outside of a tubular insulator with an internal passage extending therealong, which is conveniently used as a gas supply passage, inside which extends said conductor. In some cases though it may be preferred to have this internal gas supply passage unobstructed by any first electrode elongate portion conductor.
Without in any way wishing to restrict the scope of the present invention, it is believed that the operating principle of the above-described first form of electric discharge device of the present invention and its main advantage are based on the sequential breakdown of a series of interelectrode gaps and quick generation of an extended multi-spark surface discharge. At the initial point as the capacitance of the interelectrode gap is much less than the capacitance between the electrode and central conductor (said first electrode elongate portion), almost all voltage is applied to the first interelectrode gap (between the first and second electrodes). The sequence of successive breakdowns between the neighbouring electrodes results in the electric discharge being propagated from the first electrode portion to the second electrode via the secondary electrodes along the metal-dielectric surface until the gap between the first electrode portion and the second electrode portion is fully bridged by a high-voltage pulse discharge of the kind sometimes referred to as xe2x80x9cslipping surfacexe2x80x9d discharge. Thus the threshold of electric discharge initiation is decreased significantly and is close to the threshold of discharge between the two neighbouring electrodes e.g. the first electrode portion and the first one of the secondary electrodes. The use of the first electrode elongate portion has the particular advantage of substantially reducing, typically by 10 to 100 times, the threshold voltage required to initiate the electric spark discharge in a given device relative to that required in the same device without such a first electrode elongate portion extending alongside the electric discharge surface.
By dimensioning the internal passage in the abovedescribed type of arrangement of the first form of the invention, to have a diameter somewhat greater than that of the conductor, the passage can also serve as a conduit for gas as part of the gas delivery means. In this case the conduit may be connected to the discharge surface by a plurality of passages extending through the insulator, which may be in the form of discrete space apart passages. Alternatively there could be used an insulator with a porous or honeycombed structure providing a large multiplicity of gas passages extending therethrough and which may be interconnected with each other to a greater or lesser extent.
Advantageously only opposed electrode portions are exposed, other portions of the first and second electrodes being covered in solid dielectric insulator material. Thus in the case where the first and second electrodes have cylindrical outer surfaces, these outer surfaces are covered in solid dielectric insulator material. With such preferred forms of the device of the invention it is possible to treat reasonably efficiently liquids with relatively high conductivities, such as waste water in coastal treatment works where a greater or lesser amount of salt may be present (typical conductivities of 1400 to 2000 xcexcsiemens cmxe2x88x921), blood waste including whole blood (typical conductivities of up to around 5400 xcexcsiemens cmxe2x88x921) and waste water which has been used to wash down blood soaked areas (typical conductivities of around 2500 xcexcsiemens cmxe2x88x921) , and pot alexe2x80x94which is waste from whisky production and the like (typical conductivities of 2500 xcexcsiemens cmxe2x88x921). In more detail, with these preferred forms of device, energy losses due to current passing through the liquid are reduced. Also the voltage required to initiate the electric arc discharge may be significantly reduced. Thus it may be possible to treat liquids with relatively high conductivities, for example, up to 50,000 xcexcsiemens cmxe2x88x921, possibly up to 80,000 xcexcsiemens cmxe2x88x921.
Advantageously in the case of the first form of the invention in which annular first and second electrodes and secondary electrodes therebetween, mounted on a tubular insulator, are used, the tubular insulator is recessed underneath the opposed electrode portions so that in use of the apparatus the series of bubbles of gas is mixed with the liquid before entering the electric discharge zone between the opposed electrode portions. Furthermore where the device is used in a configuration wherein the tubular insulator on which the annular electrodes are mounted, extends generally vertically, then it is preferred that the gas supply passage outlets between opposed electrode portions (of said first and second and secondary electrodes), are disposed in close proximity to the upper ones of the respective pairs of opposed electrode portions. These modifications tend to increase the content of gas in the liquid in the electric discharge zone and to increase the residence time of gas bubbles therein, and thus have the advantage of inter alia reducing the amount of gas which needs to be pumped through the gas supply passages, thereby reducing the operating costs of the apparatus.
Advantageously the tubular insulator is provided with channel form recesses underneath said opposed electrode portions of said generally annular first and second electrodes, whereby in use of said apparatus the series of bubbles of gas is mixed with said liquid before entering said electric discharge zone between the opposed electrode portions.
In a second preferred form of the invention, the first and second electrodes are in the form of generally planar members in opposed relation, each having a plurality of apertures therein for the passage of liquid through first one and then the other of said electrodes via said electric discharge zone extending therebetween.
This second form of the invention is particularly suitable for use in continuous flow treatment of liquids for example, in public utility water supply systems. In this type of application, the first and second electrodes are mounted so as to extend substantially across the whole of the liquid is treated. The apertures in the first and second electrodes are generally formed and arranged in order to avoid creating excessive flow resistance whilst also providing a relatively substantial electrode structure resistant to erosion and low electrical resistance within the electrode structure itself. Desirably the electrodes are of a substantially uniform, e.g. honeycomb-like, structure across their transverse extent, with optionally an outer annular support or reinforcing portion.
Where required, e.g. in order to provide multiple discharge treatments of a given body of water passing through the flow passage, and/or in order to ensure that all parts of the flow stream (along its longitudinal extent) are subjected to an effective discharge treatmentxe2x80x94especially in the case of high flow rates, a plurality of opposed first and second electrodes may be used. This may be effected by the use of a series of devices mounted along the flow passage, or, more conveniently, by using a series of first and second electrode pairings connected (in parallel or series) to a common voltage supply means. In such cases the first and second electrode pairings may conveniently be spaced apart from each other by from 2 to 10 cms, advantageously from 3 to 8 cms, for example about 6 cms. The first and second electrodes in the or each pairing are generally spaced apart from each other by from 0.2 to 2 cms, preferably from 0.5 to 1 cms.
As noted above, the electrodes may have various different forms corresponding to different proportions of electrode area v. aperture area. Typically the aperture area may comprise from 30 to 98% of the total area of the electrode (corresponding to an electrode material area of from 70 to 2% of the total area). Desirably the aperture area comprises from 50 to 95%, for example, from 70 to 90%, of the total electrode area.
In a third preferred form of the invention, the first electrode is provided at a nozzle means formed and arranged for directing, in use of the apparatus, a stream of said liquid onto said second electrode, said gas supply means being formed and arranged for injecting, in use of the apparatus, said series of bubbles of gas into said stream.
With this third form of device the amount of liquid within which the electrical discharge is generated and which is treated directly, is generally rather more restricted as compared with the first and second forms of device. It is however also possible to use the more or less intense UV radiation generated by the plasma discharge within the stream of liquid, for irradiation treatment of other matter in proximity to said liquid stream. Thus, for example, a flow of liquid spaced apart from said liquid stream, may be treated.
With this third form of device, it will be appreciated that the electric discharge zone will have a somewhat different geometric configuration from that of the first and second types, and will be generally rod-shaped rather than tubular or sheet-form. Nevertheless, the form and arrangement of the electrodes as well as that of the electric discharge zone, may be varied considerably in this case also. In a simple arrangement the nozzle means is disposed substantially vertically above the second electrode so that a stream of liquid may simply fall under the influence of gravity down onto the second electrode. It is, however, also possible to use other arrangements in which a pressurized flow of liquid can be used to provide a jet of liquid onto a second electrode disposed other than vertically below a nozzle outlet, for example, to one side. The former type of arrangement is however generally preferred as it allows the use of a simple arrangement, e.g. a cup form recess, for collecting the liquid stream as it contacts the second electrode. Conveniently the nozzle itself is of an electrically conducting material in order that it can be used for or as the first electrode, though other arrangements are also possible e.g. with the first electrode as a partial or complete coating or nozzle tip attachment on a non-conducting nozzle. In the case of the second electrode this may also conveniently be integrated to a greater or lesser extent with a liquid stream collecting means, which generally has an enlarged diameter, and/or generally funnel-shaped, e.g. cup-shaped, inletxe2x80x94especially where it is desired to use an essentially closed circuit liquid supply for the liquid stream with the liquid stream being collected and returned, via suitable return passage means and flow inducing means e.g. a pump, to the nozzle. It will of course be appreciated that the return passage means should include electrical insulation means (or conveniently be of electrically insulating material) in order to electrically isolate the electrodes from each other along said passage means.
It will be appreciated that the size and form of the electric discharge zone in this third form of the invention may be varied. Thus the diameter of the nozzle outlet, as well as to some extent the liquid pressure and/or flow rate up to and through the nozzle, will affect the form and diameter of the stream of liquid. In general it is preferred to avoid high pressures and to have a relatively non-turbulent liquid flow (other than turbulence associated with the gas bubbles). Conveniently the liquid stream has a diameter of from 1 to 10 mm, desirably from 2 to 7 mm, though substantially larger diameters may also be used if desired. The separation between the first and second electrodes may conveniently be in the range from 1 to 15 cms, advantageously from 2 to 10 cms. It will be appreciated in this connection that larger separations will generally require higher voltages.
In this third form of apparatus the gas supply is generally formed and arranged so that the injected gas is substantially entrained in the liquid flow passing out of the nozzle, as a series of relatively small gas bubbles.
In general the gas delivery means comprises passage means having outlet means at or adjacent said electric discharge zone, for feeding gas to the liquid in the electric discharge zone, and inlet means for coupling, in use of the apparatus, to a gas supply means which may comprise an active device such as a compressor or pump, and/or a passive device such as a reservoir of compressed gas provided with a gas flow regulator means.
In general the gas delivery and supply means may be formed and arranged to feed gas to the liquid in said electric discharge zone at a wide variety of pressures and/or flow rates depending on various factors such as the gas used, the nature of the liquid, the size, shape and configuration of the electric discharge surface, the ambient pressure at the electric discharge surface, the high voltage pulse frequency etc. In the case of the first device form with multiple secondary electrodes, there may be used a gas feeding rate of at least 30 l hrxe2x88x921, conveniently from 30 to 100 l hrxe2x88x921 for an electric discharge surface having an area of around 25 cm2.
The gas supply means also desirably has outlet means formed and arranged to produce relatively small bubbles in the liquid, in use of the apparatus, with, for example, a bubble diameter in the range from 0.1 to 10 mm, preferably from 0.5 to 5 mm, advantageously from 0.7 to 3 mm.
Various gases may be used including more or less inert gases such as noble gases e.g. argon or nitrogen. Most conveniently and economically though there may simply be used air or oxygen which also have the advantage that they give rise to the production of ozone which can in turn allow the production of free radicals which can have further beneficial effects in the inactivation of contaminants.
The first and second electrodes may have various shapes and configurations depending inter alia on the shape and configuration of the electric discharge surface. Where this is generally annular, then the opposed portions of the first and second electrodes at respective end portions of the electric discharge surface may also conveniently be annular but this is not essential. With an annular electric discharge surface the secondary electrodes are also conveniently generally annular. Other configurations are also possible though which may have possible advantages, for example, in relation to generation of a more homogenous electric discharge and/or increased electrode life. Thus, for example, the secondary electrodes may be in the form of multiplicity of metallic or other electrically conducting particles embedded in or attached to the surface of an electrically insulating substrate. The particles may have various shapes or sizes, for example, from 0.1 mm to 5 mm in diameter, conveniently from 0.2 mm to 0.3 mm in diameter, and may be rounded, but more conveniently are angular, for example polygonal, regular or irregular, star-shaped etc. The particles are generally spaced apart at a separation not less than their diameter, but conveniently of a similar order of magnitude. This type of arrangement has the advantage of generating a much larger number of electric spark discharge routes thereby yielding a more homogeneous electric discharge zone.
In general it is desirable that there should be used an inter-electrode gap between neighbouring electrodes of at least 1 to 2 mm in order to ensure that an electric spark discharge occurs (i.e. substantially without electrical energy passage by other means). Larger gaps, for example up to 10 mm or more may be used without significantly increasing the threshold voltagexe2x80x94typically of the order of 10 kV, and are generally preferred as they provide a more extended electric spark discharge and zone. Preferably there is used an electrode gap of from 3 to 8 mm. Larger gaps may also be used but these will tend to require higher threshold voltages.
The electrical power supply control means is generally formed and arranged for conditioning a suitable available electrical power supply, in generally known manner, so as to apply across said first and second electrodes, a series of high voltage DC pulses. The voltage required to initiate and maintain an electric discharge may also be affected to some extent by various factors such as the shape and configuration of the electric discharge surface, the shape and configuration of the first and second electrodes and the secondary electrodes, as well as the number of the latter, and the liquid used. In addition we have found that by further increasing the applied voltage above the minimum threshold voltage required to initiate electrical discharge, it is possible to modify the spectral distribution of the UV radiation produced. More particularly we have found that higher voltages produce more higher energy shorter wavelength UV radiation i.e. the wavelength peak is shifted to shorter wavelengths e.g. from around 260 nm which is particularly useful for inactivating bacteria and other micro-organisms, down to around 180 nm which is more useful for removing formaldehyde and phenols. In addition at such shorter wavelengths there may be produced in water, new compounds such as H2O2 and/or O3 which have oxidising properties and may provide further sterilisation of water and/or removal or inactivation of other undesirable contaminants. In general there is used a voltage of at least 10 kV, usually from 10 kV to 50 kV.
The pulses may be applied at a range of different frequencies depending on, for example, the nature of the contaminant. Thus, for example, a higher pulsing rate may be desired where a shorter treatment time is required albeit at the cost of a higher power input requirement, or where the shock wave effect is considered particularly significant in the treatment. In general there is used a pulse rate which is sufficient to maintain a substantially continuous discharge. Conveniently there may be used a pulse rate of from 1 to 200 Hz, e.g. from 1 to 100 Hz.
In a further aspect the present invention provides a method of producing an electric discharge in a liquid comprising the steps of: providing an apparatus of the invention coupled to an electrical power supply and a gas supply means; providing a body of said liquid between said first and second electrodes; and applying a series of high voltage pulses to the first and second electrodes of the electric discharge device of said apparatus while feeding gas into said body of liquid.
In a yet further aspect the present invention provides a method of producing an electric discharge in a liquid comprising the steps of: providing a first form of apparatus of the invention coupled to an electrical power supply and a gas supply means; substantially immersing the electric discharge surface of the electric discharge device in the liquid; and applying a series of high voltage pulses to the first and second electrodes of the electric discharge device of said apparatus while feeding gas to the electric discharge surface thereof.
In yet another aspect the present invention provides a method of treating a liquid containing at least one hazardous contaminant, so as to decontaminate effectively the liquid said method producing and maintaining an electric discharge in said liquid by means of a method according to the present invention, for a period of time sufficient substantially to modify at least one said contaminant into a more benign form.
In accordance with the present invention the contaminant modification may have various different forms depending on the nature of the contaminant. In the case of micro-organisms the required modification should generally be at least sufficient to prevent effective propagation or reproduction of the micro-organism. In the case of cellular organisms such as bacteria, suitable modification may or may not involve cell disruption. Various microbial contaminants may be inactivated by means of the present invention such as E. coli, Pseudomonas aeruginosa, Cryptosporidium, etc.
With organo-chemicals suitable modification comprises reaction of the contaminant so as to convert it into another species which is environmentally benign per se and/or which is more or less readily bio-degradable into an environmentally benign form. The method of the invention may also be used effectively to inactivate inorganic contaminants such as metal salts and especially heavy metal salts, by converting them into an insoluble form which can be more or less readily precipitated out of the liquid.
The decontamination method of the invention may be carried out in either a continuous treatment mode or a batch treatment mode. In the latter case the liquid to be decontaminated should generally be confined in a chamber around the electric discharge device (or in a closed circuit including said chamber and liquid circulation means for circulating the liquid through the chamber)so that substantially the whole of the liquid is brought inside or into close proximity with the electric discharge zone for an effective period of time sufficient for substantial inactivation of the contaminants. The first preferred form of apparatus of the invention is generally most convenient for such treatment. The electric discharge itself will generally provide a more or less substantial degree of mixing of the liquid in the chamber but if desired additional mixing means may be provided. In the case of a continuous treatment mode the apparatus of the invention would generally comprise a liquid containment means having an inlet for liquid to be treated, a treatment zone inside which the electric discharge surface of the electric discharge device is disposed, an outlet for treated liquid, and liquid flow control means for regulating the flow of said liquid through the liquid containment means. The second preferred form of apparatus of the invention is generally most convenient for this.
The treatment time (discharge operation time for batch mode, or residence time in the electric discharge zone for continuous mode) will depend on various factors such as the nature of the liquid, and the nature and concentration of the contaminant(s), as well as the energy of the electric discharge. Suitable times can be readily determined by suitable trial and error for any given situation (device power and configuration, contaminant nature and loading thereof, required reduction in contaminant loading, etc.).
In order to increase efficiency in treatment of at least some contaminants of the liquid it may be desirable to control other variables as well. Thus for example in the treatment of water to inactivate micro-organisms it is preferred to use gas containing significant amounts of oxygen, and desirably to use alkaline pH values in the liquid undergoing treatment, typically from 7 to 11, preferably from 8 to 10, conveniently by means of adding alkali such as sodium hydroxide, in order to increase ozone generation and dissociation thereof to produce free radicals.
If desired, the treatments of the present invention may be combined to a greater or lesser extent with other possible treatments. Thus for example, the liquid being treated may also be heated in order to provide a more intensive and/or efficient treatment of the liquid. Also the liquid may be passed through a micro-filter.
In yet another aspect the present invention provided an apparatus suitable for use in the treatment of a liquid comprising a high voltage electric discharge device with a first electrode and a second electrode spaced apart from said first electrode and a multiplicity of spaced apart secondary electrodes distributed between opposed portions of said first and second electrodes across an electric discharge zone of said device, said discharge device including electrical power supply control means formed and arranged for applying a series of high voltage pulses across said first and second electrodes in use of the apparatus with said device connected to an electrical power supply so as to produce an extended area electrical discharge, said apparatus including a gas delivery means formed and arranged for feeding gas to said electric discharge zone in use of said apparatus with said electric discharge zone immersed in said liquid.